Communication systems that geographically reuse communication resources are known. These systems allocate a predetermined set of communication resources in one geographic area and reuse the same set of communication resources in one or more other geographic areas. This reuse technique improves communication system capacity by minimizing the number of communication resources necessary to provide communication service in a large geographic area comprised of several smaller geographic areas. As is also known, communication resources are defined by the multiplexing scheme utilized in the particular communication system. For example, with frequency division multiplexing (FDM), a communication resource may be a frequency carrier or pair of frequency carriers; whereas, with time division multiplexing (TDM), a communication resource may be a time slot or pair of time slots in one or more time frames.
In geographic reuse communication systems, signal usability of a communication resource needs to be determined in order to reliably use the communication resource. Signal usability is typically limited by the quantity of co-channel interference present on the RF channel. Co-channel interference occurs when receivers receive unwanted information signals from neighboring communication units, or base stations, transmitting on the same channel as the desired RF channel. Thus, the signal usability decreases as the co-channel interference increases.
Another alteration of the transmitted signal occurs as a result of fading. Fading occurs due to multiple reflections of the modulated signal during transmission over the RF channel. These reflections typically result from unintentional reflecting of the modulated signal from obstacles in its path, such as buildings and mountains, and may produce multiple modified replications of the modulated signal, each introducing various amplitude and phase alterations of the original signal in each new signal path. All of the modulated signal replicas form a composite signal at the input to a receiver and account for the fading. For a detailed discussion of a method for measuring signal quality that accounts for frequency selective fading refer to U.S. Pat. No. 5,170,413, entitled "Control Strategy For Reuse System Assignments And Handoff" and assigned to Motorola Inc. While this technology provides many advantages, it does not address the technological concern of estimating signal usability based on a measured approximation of co-channel interference and noise.
In order to mitigate the effects of fading, radio communication systems typically utilize diversity to enhance the signal-to-noise ratio of a modulated signal in a fading environment. Diversity techniques are incorporated in communication receivers and attempt to obtain multiple, decorrelated replicas of the transmitted signal by using multiple antennas typically spaced several wavelengths apart. Each replica is received in a diversity receiver by a respective receiver branch. Upon obtaining the multiple replications, the diversity receiver either combines the received signals from each receiver branch to form a composite signal or selects the received signal with the best signal usability from one receiver branch. Thus, by receiving multiple copies of the transmitted signal, the diversity receiver produces an output signal with a better overall signal-to-noise ratio than if only one copy of the transmitted signal were received.
To estimate signal usability of the diversity receiver's output, a variety of methods may be employed. One known method scales each received signal in each receiver branch by a scaling factor proportional to the interference in each corresponding signal path, individually determines the signal usability of each receiver branch's output signal, and sums the individual signal usabilities to form a composite signal usability of the diversity receiver's output. Although this text book method provides an accurate signal usability indication, it is not practically realizable since methods for obtaining accurate measurements, or estimates, of the interference in each signal path are not readily available. Another approach to estimating the signal usability of a receiver's output is detailed in pending U.S. patent application Ser. No. 08/069,927, entitled "A Method And Apparatus For Determining Signal Usability" and assigned to Motorola Inc. Although this technological advance provides an accurate, realizable method for determining signal usability of received signals based on co-channel interference and noise, it does not address the concern of determining signal usability of a received composite signal that is formed from multiple, simultaneously received, decorrelated signals.
Therefore a need exists for a practical method of determining signal usability of a composite signal received by a diversity receiver that is based on an estimation of RF channel interference.